30 Professor Ayrton [Jan. 24, 



sunlight, were made under certain conditions in a converging beam — 

 that is, the thicker portions of the mirror might be expected to appear 

 darker instead of brighter than the remainder. 



[Experiments were then shown of the image cast on the screen : 

 1st, when a divergent beam of light fell on the mirror ; 2nd, when 

 the beam was parallel ; 3rd, when the beam was convergent : and it 

 was seen that, 1st, the pattern appeared as bright on a dark ground ; 

 2nd, the pattern was invisible ; 3rd, the pattern appeared as dark on a 

 bright ground.] 



Again, by allowing a parallel beam of light to fall on the Japanese 

 mirror and interposing a double convex lens between the mirror and 

 the screen, we can make the image show the pattern either as bright 

 on a dark ground or as dark on a bright ground, or not at all, merely 

 by causing the screen to be : 1st, nearer the lens than the conjugate 

 focus of the mirror ; second, farther than the conjugate focus ; 3rd, 

 at the conjugate focus. [This experiment was here shown.] Now it 

 can easily be proved by simple geometrical optics that each of these 

 effects would be produced if the thicker parts of the mirror were 

 a little less convex than the remainder. [This was explained by 

 various geometrical diagrams.] And lastly, if the phenomenon was, 

 as the previous experiment would lead us to conclude, due not to 

 unequal reflecting power of the different portions of the surface of the 

 mirror, but to minute inequalities on the surface, in consequence of 

 which there is more scattering of the rays of light falling on one 

 l^ortion than on another, then since rays of light making very small 

 angles with one another do not separate perceptibly until they have 

 gone some distance, it follows, that if the screen be held very near to 

 the mirror, the apparent reflection of the back, the magical 23roperty 

 in fact, ought to become invisible. And this, also, it was shown, was 

 exactly what happened when the screen was made almost to touch the 

 polished surface. 



The lecturer then proceeded to explain why a divergent beam, 

 emitted by a bright luminous point at some fifteen feet distance from 

 the mirror, gave the best effects. 



We have, therefore, strong reasons for favouring the " inequality 

 of curvature " theory. In order, however, to make the explanation 

 quite certain, the lecturer said he had made a small concavity and a 

 small convexity on the face of one of the mirrors, by hammering with 

 a blunt tool, carefully protected with a soft cushion to avoid scratching 

 the polished surface, and he showed by experiment that the concavity 

 reflected a bright imago and the convexity a dark one when the 

 pattern on the back appeared bright, but when the light was so 

 arranged that the pattern appeared as dark on a bright ground, it was 

 the convexity which ai^peared as the bright spot, and the concavity as 

 the dark one. 



Guided by all that precedes, we are led to the undoubted con- 

 clusion, that the whole action of the magic mirror arises from the 

 tliicker portions being flatter than the remaining convex surface, and 



